Parity engages pathways within the breast epithelium that render it resistant to carcinogenesis. These pathways are potent as parity reduces the risk associated with carcinogens and inherited risk alleles preventing as many as 70,000 cases of breast cancer annually. The levels of estrogens during pregnancy are sufficient to mimic the protective effects of parity. The actions of estrogens are mediated by two estrogen receptor subtypes (ER? and ER?) that balance their competing effects on proliferation and growth suppression mediated by the p53 tumor suppressor pathway. Pregnancy also suppresses WNT/?-catenin signaling and reduces the pool of vulnerable stem cells within the mammary epithelium. However, chronic exposure to exogenous estrogens erodes the protective effects of pregnancy in rodents. During pregnancy, women are exposed to high levels of xenoestrogens (benzophenones, parabens, phthalates) that differ in their potencies for ER? and ER?. Therefore, pregnancy is a critical window during which xenoestrogens may disrupt tumor suppressor pathways and render the breast epithelium susceptible to carcinogenesis. In these experiments, we will test 4 xenoestrogens that are abundant in pregnant women and differ in potency for ER? and ER?. Parallel experiments in primary breast tissues and mouse models are used to determine if xenoestrogens interfere with the effects of pregnancy on critical tumor suppressor pathways. Specific Aim 1 will use breast epithelial cells to determine the effects of xenoestrogens on tumor suppressor pathways and the variation in responses among women. Dose-response relationships for each xenoestrogen will be determined in MCF-7 breast cancer cells expressing either ER? or both ER?+ER?. Primary cultures of normal breast epithelial cells will be used to confirm responses and define subsets of women who are highly sensitive or resistant to xenoestrogens. Specific Aim 2 compares effects of xenoestrogens in explant cultures of breast tissue and in BALB/c mice. Primary breast explants more faithfully reflect signaling within breast tissue and complement results from Aim 1. Parallel treatments in mice that are particularly sensitive to estrogens (BALB/c) will validate the in vivo model and determine effects of chronic exposures to xenoestrogens on tumor suppressor pathways. Specific Aim 3 will test the effects of xenoestrogens on tumorigenesis. The ability of xenoestrogens to cause progression of premalignant cells will be tested using transplants of CD?Geo cells. The ability of xenoestrogens to reverse the protective effect of parity will be tested in mice bearing transplants of Trp53+/- mammary epithelium. Community Engagement: The Community Partners provide a means to dynamically communicate the concerns of the community and results from research. Importantly, the experiments are designed to identify a subset of women who may be especially vulnerable to xenoestrogens and demonstrate the relevance of the mouse models to human exposures.